What do you think Malthael’s plans are for the Black Soulstone?

With the recent release of the Reaper of Souls Friends and Family Beta, we’ve seen a lot of discussion focused on gameplay systems, class balance, item balance, difficulty tuning, and more. In the wake of those more technical conversations, there've also been a number of story-related inquiries raised concerning the Malthael and his return to Sanctuary.

So, for our latest “Question from Sanctuary” (our new name for this particular blog series), we want to know:

Why do you think Malthael took the Black Soulstone from Tyrael? And what do you think he intends to do with it?